INITIAL RESPONSE OF STEP-POOL STREAMS TO WILDFIRE

Although knowledge of the hydrological and geomorphological impacts of wildfire has advanced significantly in recent decades, the effects of wildfire remain difficult to predict. These effects vary with characteristics of the fire and watershed affected, as well as intensities and durations of precipitation. Temporal windows for observing post-fire effects are also typically short, extending just a few years after fire. These challenges are especially magnified in deciphering the response of step-pool streams that characterize steep rugged terrains. Because step-pool systems are stable features that respond to extreme events with recurrence intervals often exceeding 50 years, opportunities for observing such responses are especially rare. This study documents the response of step-pool streams within Pike National Forest, Colorado, following the 2012 Waldo Canyon Fire, one of several recent wildfires that spread across the Colorado Front Range. Within two months after the June/July burn, before significant channel changes occurred, baseline data were collected at 14 channel reaches within Williams Canyon and the Camp Creek watershed and three reference reaches outside the burn area. Channel morphology was documented with longitudinal profile and cross-sectional surveys and terrestrial LiDAR scanning; pebble counts and sediment samples enabled assessment of sediment caliber. Analysis of benthic macroinvertebrates further provided indication of ecological quality. Repeat measurements following the first geomorphologically significant event in July 2013 revealed a variable response. In Williams Canyon, post-fire flash floods widened channels and lowered river beds by as much as one meter. The flow events obliterated the step-pool structure and resulted in featureless plane beds and riffles. Changes in ecological character accompanied the conversion of channel morphology, including a reduction in species richness and diversity to near zero. Despite these dramatic changes, the study reaches in the neighboring Camp Creek basin remain largely unaltered in response to the same storms. These results underscore the difficulties of generalizing and predicting the impacts of wildfire, as well as the importance of repeated observations over the long term.